In systems employing channel reuse schemes, it is a common problem to determine whether one is using the channel intended or an unintended reuse of that channel. In cellular radiotelephone systems exhibiting geographical frequency reuse, the problem is worse because cellular radiotelephones are not fully capable of rejecting not only the same frequency reused at the geographic separation distance, but also insufficiently capable of rejecting adjacent frequencies within the reuse perimeter. U.S. Pat. No. 4,128,740 to Graziano, assigned to the instant assignee, is useful to demonstrate the point. FIG. 4 of Graziano (substantially reproduced here as FIG. 1) illustrates a conventional cellular frequency reuse plan commonly known as the seven-cell repeat pattern. It consists of a central cell (1) ringed in a clockwise direction beginning at 3-o'clock by six ring cells (2-7), completing a seven-cell pattern. That seven-cell pattern is then itself surrounded by six other identical seven-cell patterns. Thus, the frequencies operating in each cell are geographically reused at a geographical reuse separation distance. It is this geographical frequency reuse that provides the (frequency) channel capacity to allow a large number of radiotelephone users to share a limited amount of (frequency) spectrum. However, the neat geographical reuse separation illustrated is not quite so neat when local propagation conditions such as antenna height, buildings and obstructions, etc., are taken into account. Accordingly, additional protection is provided against co-channel (same frequency) interference in the signalling by allocating seven differentiating codes to each of the surrounding patterns. This signalling provides some protection in that a radiotelephone is able to differentiate the frequency on which he is speaking from an interferer of the same frequency at the reuse separation distance but having a different code. In conventional analog cellular systems, these codes are Supervisory Audio Tones (SAT) and in proposed narrower band, split-channel systems, known as Digital SAT or DSAT.
The problem solved by the instant invention is that there is insufficient rejection in radiotelephones to fully reject adjacent channels just one frequency above or below the one of interest.
In his patent (U.S. Pat. No. 4,128,740), Graziano teaches that greater frequency reuse (and therefore spectrum utilization) can be obtained with narrow beam, sectored antennas. The resulting four-cell pattern is still surrounded by six reuses of that same four cell pattern, advantageously employing seven DSATs for improved signalling protection against co-channel interference. FIG. 9 of Graziano (substantially reproduced here as FIG. 2) illustrates a frequency plan for his "four-cell" pattern. It illustrates how 208 frequency channels are divided among 4 cells, each having 6 sectors, for a total of 24 sectors (CS1-CS24). Thus, each sector has one supervisory frequency (S1-S24) and either 7 or 8 voice frequency channels, allowing many simultaneous conversations in each sector. The problem is that, although any given frequency being used in Sector 3 of a first pattern can be differentiated from that same co-channel frequency in Sector 3 of the pattern behind it by its DSAT, there is no protection for a radiotelephone user in Sector 3 using frequency 196 from the next higher adjacent frequency 197 directly behind it in Sector 4, or from the next lower adjacent frequency 195 in Sector 2 behind and to the side of it.
This invention takes as its object to overcome these shortcomings and to realize certain advantages presented below.